Add new parameters for IDENTITY start/increment in mssql

Add new parameters for IDENTITY start/increment in mssql

Deprecated the use of :class:`.Sequence` with SQL Server in order to affect
the "start" and "increment" of the IDENTITY value, in favor of new
parameters ``mssql_identity_start`` and ``mssql_identity_increment`` which
set these parameters directly.  :class:`.Sequence` will be used to generate
real ``CREATE SEQUENCE`` DDL with SQL Server in a future release.

Fixes: #4362
Change-Id: I1e69378c5c960ff0bc28137c923589692f1a918f